1. Field of the Invention
The present invention relates to a control device for a continuously variable transmission, which is used as a vehicle power transmission device, and a method of controlling the continuously variable transmission.
2. Discussion of the Prior Art
In vehicles, a drive force generated by an internal combustion engine is transmitted to the driving wheels, through a transmission. The transmission changes the drive force to be transmitted to the driving wheels in accordance with the vehicle running conditions changing in a broad range, whereby bringing out the best performances of the internal combustion engine. The transmission is generally categorized into a gear transmission in which a speed change ratio is stepwise changed by selectively switching gears of the gear train, and a continuously variable transmission (CVT) in which a speed change ratio is continuously changed by changing the radius of rotation of a belt wound around pulleys.
In some types of CVT, a belt is wound around a drive pulley and a driven pulley. The width of a groove between the pulley parts of each pulley is changed by an oil pressure, thereby to change the radius of rotation of the belt. With the change of the rotation radius, a speed change ratio is continuously changed. In the CVT, a control device controls an oil pressure according to a control mode selected from among various control modes, thereby to change the speed change ratio. The CVT is provided with an oil pressure clutch. In the clutch, a coupling state and a decoupling state are selectively set up for the various control modes by controlling an oil pressure by means of the control device. For the details of the CVT, reference is made to Japanese Patent Laid-Open Publication Nos. Sho. 57-186656, 59-43249, 59-77159, and 61-233256.
In the CVT, when the controls of a line pressure as an oil pressure and a clutch pressure start immediately after the control mode is shifted from a hold mode to a start mode, with the calculating operation of the control device and retardation of increasing an oil pressure up to a predetermined oil pressure, the control system of the vehicle often fails to secure a line pressure necessary for the clutch pressure control.
Japanese Patent Laid-Open Publication No. Hei. 1-119433 discloses a technique to secure a sufficient line pressure. In the technique, the control of the clutch pressure in a start mode is delayed in its starting, when the control mode is shifted from the hold mode to the start mode.
In the internal combustion engine carried on the vehicle, the number of idling revolutions is, for example, 700 to 900 rpm in a complete warming-up state. In the conventional control method, when the vehicle is manipulated for start under the condition that the internal combustion engine runs at the idling speed, the control mode is shifted from the hold mode to the start mode even if the engine speed is equal to the idling speed. A line pressure as an oil pressure that is required in the start mode is at least 15 kg/cm.sup.2.
At low engine speeds, e.g., idling speeds, a discharge flow from the oil pump is small. Therefore, it is impossible to gain an oil pressure necessary for the control for vehicle start. Accordingly, the CVT can be unsatisfactorily controlled, so that the running feeling at the start of vehicle is impaired.
As shown in FIGS. 1 through 4, in a low speed region (A) of the engine speeds NE, the discharge flow of the oil pump is insufficient. Under this condition, there occurs a region where a line pressure cannot follow a line pressure solenoid duty OPWLIN. Such a region will impair the running feeling at the start of vehicle. In the start mode, the line pressure solenoid duty OPWLIN becomes high to increase the load to the internal combustion engine. This impedes the increase of the engine speed. As a result, the running feeling is further deteriorated at the start of vehicle.
This will be described in more detail. In FIG. 4, a driver pushes his foot down on the accelerator pedal. Then, the driver demand switch DDT is turned on. Immediately after the switch is turned on, the control mode is shifted from the hold mode HLD to the normal start mode NST. When the driver demand switch DDT is off, the internal combustion engine is in an idling state, and the engine speed is within the range from 700 to 900 rpm . Accordingly, the engine speed keeps the idling speed for a period of time after the driver demand switch DDT is turned on.
The control system of the vehicle recognizes the driver's intention to start a vehicle on the basis of only the turn-on of the driver demand switch DDT. However, there is a case where the on-state of the switch DDT does not indicate the driver's intention to start a vehicle. For example, when a driver merely puts his foot on the accelerator pedal, the switch DDT is turned on like the rest. The control system of the vehicle mistakenly recognizes that the driver will start the vehicle.
Additionally, increase of the line pressure results in increase of the load to the internal combustion engine. If the control system is designed so that the line pressure is increased at low engine speeds NE, the engine sometimes fails to reach an engine speed NE high enough to control the normal start mode NST. Particularly in the internal combustion engine of low power performances, such a situation tends to occur sensitive to nonuniformity of the engine performances of the engines used. This also impairs the driving feeling at the start of vehicle.
As shown in FIG. 1, in the low region A of the engine speed NE, the amount of discharge flow of the oil pump is not sufficient. At the low engine speeds A, even if the line pressure solenoid duty OPWLINE is increased, the line pressure fails to follow the increasing line-pressure solenoid duty OPWLIN, as shown in FIGS. 2 and 3. This causes the unsatisfactory driving feeling at the start of vehicle. The speed change ratio of the CVT is controlled in a feedback mode so that actual engine speeds are equal to desired engine speeds that depend on the running conditions of a vehicle. The control gains are determined so that the actual engine speeds follow the desired engine speeds. At the time of rapid acceleration or deceleration, greater variation of control factors transiently occurs. With this transient great variation, an amount of manipulation is too large to be covered by the follow-up capability of the CVT. Under this condition, the belt will slip. Increase of belt gripping force may remove the transient belt slip. The excessive gripping force constantly applied brings about reduction of the lifetime of the belt and increase of fuel consumption, however.